The other side of the matter

We know that exist a particular type of matter: the antimatter. Antimatter is composed by antiparticles. An antiparticle has the same mass as the corresponding particle but has opposite charge. And luckily for us antimatter is substantialy absent from our universe: indeed the interaction between matter and antimatter leads to the annihilation process, with the disappearance of particle and antiparticle and energy production. So, if in the universe there were the same amount of matter and antimatter, it would be filled exclusively with radiation. For this reason it is particularly interesting understand where this asymmetry originates: we know that would be a symmetry violation in some place and time of the universe, and the T2K experiment in Japan tested neutrinos' oscillations, in particular the oscillation from muonic to electronic neutrino. The results of ten years of data say that 90 neutrinos and only 15 antineutrinos were caught oscillating from muonic to electronic: different numbers mean violated symmetry.
The most interesting detail is that the experimental result doesn't exclude an interesting idea about an anti-universe that exists at the other side of the Big Bang.

Abe, K., Akutsu, R., Ali, A., Alt, C., Andreopoulos, C., Anthony, L., ... & Ashida, Y. (2020). Constraint on the Matter-Antimatter Symmetry-Violating Phase in Neutrino Oscillations. Nature volume 580, pages 339–344. doi:10.1038/s41586-020-2177-0

Boyle, L., Finn, K., & Turok, N. (2018). C P T-Symmetric Universe. Physical review letters, 121(25), 251301. doi:10.1103/PhysRevLett.121.251301